The present invention relates to an imaging lens for imaging and focusing an image of an object on an image plane, and relates to a compact lens system having a small number of lens elements and a lens system having a large number of lens elements with high performance.
Imaging lenses which are to be used in equipment that requiring high resolution must have comparable imaging performance as compared to conventional photographic lenses such as those used with single-lens reflex cameras. However, if imaging lenses are to be used with equipment such as handy picturephones which comparatively low resolution, they should satisfy more of the requirement for compactness than imaging performance and it is necessary to design a most compact system using the smallest possible number of lens elements. Imaging lens systems that have been commonly used in these applications are composed of three glass lens elements having spherical surfaces. As long as spherical lens elements are used, it is difficult to further reduce their number without sacrificing the imaging performance of the system.
FIG. 51 is a simplified cross section of a prior art imaging lens system that comprises three lens elements 1, 2 and 3. FIG. 52 shows lateral aberrations caused in the lens system shown in FIG. 51. Shown by 4 in FIG. 51 is cover glass. FIG. 53 is a simplified cross section of another prior art imaging lens system comprising only one aspheric lens element 5 to achieve reduction in the number of lens elements used. However, as is clear from FIG. 54, this system experiences considerable deterioration in performance, particularly in the marginal area. Although the imaging lens shown in FIG. 51 meets the requirement of the imaging lens performance, the number of the lens elements is increased. Inversely, although the lens shown in FIG. 53 has the minimum small number of elements, the performance thereof degrades in the marginal portion.